-// Rust-101, Part 06: Lifetimes, Testing
-// =====================================
+// Rust-101, Part 06: Copy, Lifetimes
+// ==================================
-use std::cmp;
-use std::ops;
-use std::fmt;
+// We continue to work on our `BigInt`, so we start by importing what we already established.
use part05::BigInt;
-
-impl PartialEq for BigInt {
- fn eq(&self, other: &BigInt) -> bool {
+// With `BigInt` being about numbers, we should be able to write a version of `vec_min`
+// that computes the minimum of a list of `BigInt`. First, we have to write `min` for `BigInt`.
+impl BigInt {
+ fn min_try1(self, other: Self) -> Self {
+ //@ Just to be sure, we first check that both operands actually satisfy our invariant. `debug_assert!` is a
+ //@ macro that checks that its argument (must be of type `bool`) is `true`, and panics otherwise. It gets
+ //@ removed in release builds, which you do with `cargo build --release`.
debug_assert!(self.test_invariant() && other.test_invariant());
- self.data == other.data
+ // Now our assumption of having no trailing zeros comes in handy:
+ // If the lengths of the two numbers differ, we already know which is larger.
+ if self.data.len() < other.data.len() {
+ self
+ } else if self.data.len() > other.data.len() {
+ other
+ } else {
+ // **Exercise 06.1**: Fill in this code.
+ unimplemented!()
+ }
}
}
-fn call_eq() {
- let b1 = BigInt::new(13);
- let b2 = BigInt::new(37);
- println!("b1 == b1: {} ; b1 == b2: {}; b1 != b2: {}", b1 == b1, b1 == b2, b1 != b2);
+// Now we can write `vec_min`.
+fn vec_min(v: &Vec<BigInt>) -> Option<BigInt> {
+ let mut min: Option<BigInt> = None;
+ // If `v` is a shared reference to a vector, then the default for iterating over it is to call `iter`, the iterator that borrows the elements.
+ for e in v {
+ let e = e.clone();
+ min = Some(match min { /*@*/
+ None => e, /*@*/
+ Some(n) => e.min_try1(n) /*@*/
+ }); /*@*/
+ }
+ min
}
+//@ Now, what's happening here? Why do we have to to make a full (deep) copy of `e`, and why did we not
+//@ have to do that in our previous version?
+//@
+//@ The answer is already hidden in the type of `vec_min`: `v` is just borrowed, but
+//@ the Option<BigInt> that it returns is *owned*. We can't just return one of the elements of `v`,
+//@ as that would mean that it is no longer in the vector! In our code, this comes up when we update
+//@ the intermediate variable `min`, which also has type `Option<BigInt>`. If you replace get rid of the
+//@ `e.clone()`, Rust will complain "Cannot move out of borrowed content". That's because
+//@ `e` is a `&BigInt`. Assigning `min = Some(*e)` works just like a function call: Ownership of the
+//@ underlying data is transferred from `e` to `min`. But that's not allowed, since
+//@ we just borrowed `e`, so we cannot empty it! We can, however, call `clone` on it. Then we own
+//@ the copy that was created, and hence we can store it in `min`. <br/>
+//@ Of course, making such a full copy is expensive, so we'd like to avoid it. We'll come to that in the next part.
+// ## `Copy` types
+//@ But before we go there, I should answer the second question I brought up above: Why did our old `vec_min` work?
+//@ We stored the minimal `i32` locally without cloning, and Rust did not complain. That's because there isn't
+//@ really much of an "ownership" when it comes to types like `i32` or `bool`: If you move the value from one
+//@ place to another, then both instances are "complete". We also say the value has been *duplicated*. This is in
+//@ stark contrast to types like `Vec<i32>`, where moving the value results in both the old and the new vector to
+//@ point to the same underlying buffer. We don't have two vectors, there's no proper duplication.
+//@
+//@ Rust calls types that can be easily duplicated `Copy` types. `Copy` is another trait, and it is implemented for
+//@ types like `i32` and `bool`. Remember how we defined the trait `Minimum` by writing `trait Minimum : Copy { ...`?
+//@ This tells Rust that every type that implements `Minimum` must also implement `Copy`, and that's why the compiler
+//@ accepted our generic `vec_min` in part 02. `Copy` is the first *marker trait* that we encounter: It does not provide
+//@ any methods, but makes a promise about the behavior of the type - in this case, being duplicable.
-impl fmt::Debug for BigInt {
- fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
- self.data.fmt(f)
- }
-}
+//@ If you try to implement `Copy` for `BigInt`, you will notice that Rust
+//@ does not let you do that. A type can only be `Copy` if all its elements
+//@ are `Copy`, and that's not the case for `BigInt`. However, we can make
+//@ `SomethingOrNothing<T>` copy if `T` is `Copy`.
+use part02::{SomethingOrNothing,Something,Nothing};
+impl<T: Copy> Copy for SomethingOrNothing<T> {}
+//@ Again, Rust can generate implementations of `Copy` automatically. If
+//@ you add `#[derive(Copy,Clone)]` right before the definition of `SomethingOrNothing`,
+//@ both `Copy` and `Clone` will automatically be implemented.
+//@ ## An operational perspective
+//@ Instead of looking at what happens "at the surface" (i.e., visible in Rust), one can also explain
+//@ ownership passing and how `Copy` and `Clone` fit in by looking at what happens on the machine. <br/>
+//@ When Rust code is executed, passing a value (like `i32` or `Vec<i32>`) to a function will always
+//@ result in a shallow copy being performed: Rust just copies the bytes representing that value, and
+//@ considers itself done. That's just like the default copy constructor in C++. Rust, however, will
+//@ consider this a destructive operation: After copying the bytes elsewhere, the original value must
+//@ no longer be used. After all, the two could now share a pointer! If, however, you mark a type `Copy`,
+//@ then Rust will *not* consider a move destructive, and just like in C++, the old and new value
+//@ can happily coexist. Now, Rust does not allow you to overload the copy constructor. This means that
+//@ passing a value around will always be a fast operation, no allocation or any other kind of heap access
+//@ will happen. In the situations where you would write a copy constructor in C++ (and hence
+//@ incur a hidden cost on every copy of this type), you'd have the type *not* implement `Copy`, but only
+//@ `Clone`. This makes the cost explicit.
+// ## Lifetimes
+//@ To fix the performance problems of `vec_min`, we need to avoid using `clone`. We'd like
+//@ the return value to not be owned (remember that this was the source of our need for cloning), but *borrowed*.
+//@ In other words, we want to return a shared reference to the minimal element.
-impl BigInt {
- pub fn inc(&mut self, mut by: u64) {
- panic!("Not yet implemented.");
+//@ The function `head` demonstrates how that could work: It returns a reference to the first element of a vector if it is non-empty.
+//@ The type of the function says that it will either return nothing, or it will return a borrowed `T`.
+//@ We can then obtain a reference to the first element of `v` and use it to construct the return value.
+fn head<T>(v: &Vec<T>) -> Option<&T> {
+ if v.len() > 0 {
+ Some(&v[0]) /*@*/
+ } else {
+ None
}
}
-
-
-#[test]
-fn test_inc() {
- let mut b = BigInt::new(1337);
- b.inc(1337);
- assert!(b == BigInt::new(1337 + 1337));
-
- b = BigInt::new(0);
- assert_eq!(b, BigInt::from_vec(vec![0]));
- b.inc(1 << 63);
- assert_eq!(b, BigInt::from_vec(vec![1 << 63]));
- b.inc(1 << 63);
- assert_eq!(b, BigInt::from_vec(vec![0, 1]));
- b.inc(1 << 63);
- assert_eq!(b, BigInt::from_vec(vec![1 << 63, 1]));
- b.inc(1 << 63);
- assert_eq!(b, BigInt::from_vec(vec![0, 2]));
+// Technically, we are returning a pointer to the first element. But doesn't that mean that callers have to be
+// careful? Imagine `head` would be a C++ function, and we would write the following code.
+/*
+ int foo(std::vector<int> v) {
+ int *first = head(v);
+ v.push_back(42);
+ return *first;
+ }
+*/
+//@ This is very much like our very first motivating example for ownership, at the beginning of part 04:
+//@ `push_back` could reallocate the buffer, making `first` an invalid pointer. Again, we have aliasing (of `first`
+//@ and `v`) and mutation. But this time, the bug is hidden behind the call to `head`. How does Rust solve this? If we translate
+//@ the code above to Rust, it doesn't compile, so clearly we are good - but how and why?
+//@ (Notice that have to explicitly assert using `unwrap` that `first` is not `None`, whereas the C++ code
+//@ above would silently dereference a `NULL`-pointer. But that's another point.)
+fn rust_foo(mut v: Vec<i32>) -> i32 {
+ let first: Option<&i32> = head(&v);
+ /* v.push(42); */
+ *first.unwrap()
}
+//@ To give the answer to this question, we have to talk about the *lifetime* of a reference. The point is, saying that
+//@ you borrowed your friend a `Vec<i32>`, or a book, is not good enough, unless you also agree on *how long*
+//@ your friend can borrow it. After all, you need to know when you can rely on owning your data (or book) again.
+//@
+//@ Every reference in Rust has an associated lifetime, written `&'a T` for a reference with lifetime `'a` to something of type `T`. The full
+//@ type of `head` reads as follows: `fn<'a, T>(&'a Vec<T>) -> Option<&'a T>`. Here, `'a` is a *lifetime variable*, which
+//@ represents for how long the vector has been borrowed. The function type expresses that argument and return value have *the same lifetime*.
+//@
+//@ When analyzing the code of `rust_foo`, Rust has to assign a lifetime to `first`. It will choose the scope
+//@ where `first` is valid, which is the entire rest of the function. Because `head` ties the lifetime of its
+//@ argument and return value together, this means that `&v` also has to borrow `v` for the entire duration of
+//@ the function `rust_foo`. So when we try to create a unique reference to `v` for `push`, Rust complains that the two references (the one
+//@ for `head`, and the one for `push`) overlap, so neither of them can be unique. Lucky us! Rust caught our mistake and made sure we don't crash the program.
+//@
+//@ So, to sum this up: Lifetimes enable Rust to reason about *how long* a reference is valid, how long ownership has been borrowed. We can thus
+//@ safely write functions like `head`, that return references into data they got as argument, and make sure they
+//@ are used correctly, *while looking only at the function type*. At no point in our analysis of `rust_foo` did
+//@ we have to look *into* `head`. That's, of course, crucial if we want to separate library code from application code.
+//@ Most of the time, we don't have to explicitly add lifetimes to function types. This is thanks to *lifetime elision*,
+//@ where Rust will automatically insert lifetimes we did not specify, following some [simple, well-documented rules](https://doc.rust-lang.org/stable/book/lifetimes.html#lifetime-elision).
+
+//@ [index](main.html) | [previous](part05.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part06.rs) | [next](part07.html)
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